Brain-specific protein C activation during carotid artery occlusion in humans

New Directions in Infection-Associated Ischemic Stroke

The relationship between infections and stroke has not been fully characterized, probably delaying the development of specific treatments. This narrative review addresses mechanisms of stroke linked to infections, including hypercoagulability, endothelial dysfunction, vasculitis, and impaired thrombolysis. SARS-CoV-2, the virus that causes COVID-19, may promote the development of stroke, which may represent its most severe neurological complication. The development of specific therapies for infection-associated stroke remains a profound challenge. Perhaps the most important remaining issue is the distinction between infections that trigger a stroke versus infections that are truly incidental. This distinction likely requires the establishment of appropriate biomarkers, candidates of which are elevated levels of fibrin D-dimer and anticardiolipin/antiphospholipid antibodies. These candidate biomarkers might have potential use in identifying pathogenic infections preceding stroke, which is a precursor to establishing specific therapies for this syndrome.

Collateral Circulation Augmentation and Neuroprotection as Adjuvant to Mechanical Thrombectomy in Acute Ischemic Stroke

PURPOSE OF THE REVIEW

Mechanical thrombectomy (MT)-mediated endovascular recanalization has dramatically transformed treatment and outcomes after acute ischemic stroke caused by a large vessel occlusion (LVO). Current guidelines recommend MT up to 24 hours from stroke onset in carefully selected patients based on favorable clinical and imaging parameters. Despite optimal patient selection and low complication rates with current recanalization technology, approximately 1 in 2 patients with LVO stroke do not achieve functional independence at 3 months. This ceiling effect of MT efficacy may be explained by ischemic core expansion into the ischemic penumbra before recanalization and neuronal loss occurring after recanalization. Factors affecting the efficacy of MT, or the degree of irreversible injury, include time from symptom onset to recanalization, collateral circulation status, and differences in neuronal vulnerability. The purpose of this brief review is to discuss potential targets for neuroprotection, present and future potential pharmacologic and nonpharmacologic agents, and the data available in the literature.

RECENT FINDINGS

In experimental ischemia models, several authors reported that pharmacologic and nonpharmacologic agents are able to slow the progression of ischemic core expansion. However, in the era of unsuccessful recanalization of the occluded artery, several neuroprotective agents that were promising in the preclinical stage failed phase II/III clinical trials.

SUMMARY

Providing neuroprotection before and after recanalization of an LVO may play an important role in improving outcomes in the era of MT. Neuroprotection is classically defined as a process that results in the salvage, recovery, or regeneration of neuronal (and other supporting CNS cell) structure or function. The advent of successful recanalization of acute LVO by MT in the majority of patients may spur the growth of effective neuroprotection.

Parasite histones are toxic to brain endothelium and link blood barrier breakdown and thrombosis in cerebral malaria

Microvascular thrombosis and blood-brain barrier (BBB) breakdown are key components of cerebral malaria (CM) pathogenesis in African children and are implicated in fatal brain swelling. How Plasmodium falciparum infection causes this endothelial disruption and why this occurs, particularly in the brain, is not fully understood. In this study, we have demonstrated that circulating extracellular histones, equally of host and parasite origin, are significantly elevated in CM patients. Higher histone levels are associated with brain swelling on magnetic resonance imaging. On postmortem brain sections of CM patients, we found that histones are colocalized with P falciparum-infected erythrocytes sequestered inside small blood vessels, suggesting that histones might be expelled locally during parasite schizont rupture. Histone staining on the luminal vascular surface colocalized with thrombosis and leakage, indicating a possible link between endothelial surface accumulation of histones and coagulation activation and BBB breakdown. Supporting this, patient sera or purified P falciparum histones caused disruption of barrier function and were toxic to cultured human brain endothelial cells, which were abrogated with antihistone antibody and nonanticoagulant heparin. Overall, our data support a role for histones of parasite and host origin in thrombosis, BBB breakdown, and brain swelling in CM, processes implicated in the causal pathway to death. Neutralizing histones with agents such as nonanticoagulant heparin warrant exploration to prevent brain swelling in the development or progression of CM and thereby to improve outcomes.

Activated protein C, protease activated receptor 1, and neuroprotection

Protein C is a plasma serine protease zymogen whose active form, activated protein C (APC), exerts potent anticoagulant activity. In addition to its antithrombotic role as a plasma protease, pharmacologic APC is a pleiotropic protease that activates diverse homeostatic cell signaling pathways via multiple receptors on many cells. Engineering of APC by site-directed mutagenesis provided a signaling selective APC mutant with 3 Lys residues replaced by 3 Ala residues, 3K3A-APC, that lacks >90% anticoagulant activity but retains normal cell signaling activities. This 3K3A-APC mutant exerts multiple potent neuroprotective activities, which require the G-protein-coupled receptor, protease activated receptor 1. Potent neuroprotection in murine ischemic stroke models is linked to 3K3A-APC-induced signaling that arises due to APC's cleavage in protease activated receptor 1 at a noncanonical Arg46 site. This cleavage causes biased signaling that provides a major explanation for APC's in vivo mechanism of action for neuroprotective activities. 3K3A-APC appeared to be safe in ischemic stroke patients and reduced bleeding in the brain after tissue plasminogen activator therapy in a recent phase 2 clinical trial. Hence, it merits further clinical testing for its efficacy in ischemic stroke patients. Recent studies using human fetal neural stem and progenitor cells show that 3K3A-APC promotes neurogenesis in vitro as well as in vivo in the murine middle cerebral artery occlusion stroke model. These recent advances should encourage translational research centered on signaling selective APC's for both single-agent therapies and multiagent combination therapies for ischemic stroke and other neuropathologies.

Activated protein C promotes neuroprotection: mechanisms and translation to the clinic

Activated protein C (APC) is a plasma serine protease that is capable of antithrombotic, anti-inflammatory, anti-apoptotic, and cell-signaling activities. Animal injury studies show that recombinant APC and some of its mutants are remarkably therapeutic for a wide range of injuries. In particular, for neurologic injuries, APC reduces damage caused by ischemia/reperfusion in the brain, by acute brain trauma, and by chronic neurodegenerative conditions. For these neuroprotective effects, APC requires endothelial cell protein C receptor. APC activates cell signaling networks with alterations in gene expression profiles by activating protease activated receptors 1 and 3. To minimize APC-induced bleeding risk, APC variants were engineered to lack > 90% anticoagulant activity but retain normal cell signaling. The neuroprotective APC mutant, 3K3A-APC which has Lys191-193 mutated to Ala191-193, is very neuroprotective and it is currently in clinical trials for ischemic stroke.

2016 Scientific Sessions Sol Sherry Distinguished Lecturer in Thrombosis: Thrombotic Stroke: Neuroprotective Therapy by Recombinant-Activated Protein C

APC (activated protein C), derived from the plasma protease zymogen, is antithrombotic and anti-inflammatory. In preclinical injury models, recombinant APC provides neuroprotection for multiple injuries, including ischemic stroke. APC acts directly on brain endothelial cells and neurons by initiating cell signaling that requires multiple receptors. Two or more major APC receptors mediate APC's neuroprotective cell signaling. When bound to endothelial cell protein C receptor, APC can cleave protease-activated receptor 1, causing biased cytoprotective signaling that reduces ischemia-induced injury. Pharmacological APC alleviates bleeding induced by tissue-type plasminogen activator in murine ischemic stroke studies. Remarkably, APC's signaling promotes neurogenesis. The signaling-selective recombinant variant of APC, 3K3A-APC, was engineered to lack most of the APC's anticoagulant activity but retain APC's cell signaling actions. Recombinant 3K3A-APC is in ongoing National Institutes of Health (NIH)-funded clinical trials for ischemic stroke.

Physiological cerebrovascular remodeling in response to chronic mild hypoxia: A role for activated protein C

Activated protein C (APC) is a serine protease that promotes favorable changes in vascular barrier integrity and post-ischemic angiogenic remodeling in animal models of ischemic stroke, and its efficacy is currently being investigated in clinical ischemic stroke trials. Interestingly, application of sub-clinical chronic mild hypoxia (CMH) (8% O2) also promotes angiogenic remodeling and increased tight junction protein expression, suggestive of enhanced blood-brain barrier (BBB) integrity, though the role of APC in mediating the influence of CMH has not been investigated. To examine this potential link, we studied CMH-induced cerebrovascular remodeling after treating mice with two different reagents: (i) a function-blocking antibody that neutralizes APC activity, and (ii) exogenous recombinant murine APC. While CMH promoted endothelial proliferation, increased vascular density, and upregulated the angiogenic endothelial integrins α5β1 and αvβ3, these events were almost completely abolished by functional blockade of APC. Consistent with these findings, addition of exogenous recombinant APC enhanced CMH-induced endothelial proliferation, expansion of total vascular area and further enhanced the CMH-induced right-shift in vessel size distribution. Taken together, our findings support a key role for APC in mediating physiological remodeling of cerebral blood vessels in response to CMH.

Cytoprotective-selective activated protein C therapy for ischaemic stroke

Despite years of research and efforts to translate stroke research to clinical therapy, ischaemic stroke remains a major cause of death, disability, and diminished quality of life. Primary and secondary preventive measures combined with improved quality of care have made significant progress. However, no novel drug for ischaemic stroke therapy has been approved in the past decade. Numerous studies have shown beneficial effects of activated protein C (APC) in rodent stroke models. In addition to its natural anticoagulant functions, APC conveys multiple direct cytoprotective effects on many different cell types that involve multiple receptors including protease activated receptor (PAR) 1, PAR3, and the endothelial protein C receptor (EPCR). Application of molecular engineered APC variants with altered selectivity profiles to rodent stroke models demonstrated that the beneficial effects of APC primarily require its cytoprotective activities but not its anticoagulant activities. Extensive basic, preclinical, and clinical research provided a compelling rationale based on strong evidence for translation of APC therapy that has led to the clinical development of the cytoprotective-selective APC variant, 3K3A-APC, for ischaemic stroke. Recent identification of non-canonical PAR1 and PAR3 activation by APC that give rise to novel tethered-ligands capable of inducing biased cytoprotective signalling as opposed to the canonical signalling provides a mechanistic explanation for how APC-mediated PAR activation can selectively induce cytoprotective signalling pathways. Collectively, these paradigm-shifting discoveries provide detailed insights into the receptor targets and the molecular mechanisms for neuroprotection by cytoprotective-selective 3K3A-APC, which is currently a biologic drug in clinical trials for ischaemic stroke.

Neurotoxicity of the anticoagulant-selective E149A-activated protein C variant after focal ischemic stroke in mice

Wild type (WT) activated protein C (APC) and cytoprotective-selective APC variants such as 3K3A-APC (<10% anticoagulant but normal cytoprotective activity) are neuroprotective in murine focal ischemic stroke models. Here we compared the neuroprotective effects of the anticoagulant-selective E149A-APC variant (>3-fold increased anticoagulant activity but defective cytoprotective activities) to those of the cytoprotective-selective 5A-APC variant (<10% anticoagulant activity). After transient distal middle cerebral artery occlusion, mice received a vehicle, E149A-APC or 5A-APC at 0.2mg/kg at 4h after stroke. Treatment with 5A-APC was neuroprotective, as it improved performance on forelimb use asymmetry test and foot fault test (P<0.05), reduced by 48% and 50% the infarct and edema volumes, respectively (P<0.05), and was not associated with an increased risk of bleeding as indicated by normal hemoglobin levels in the ischemic brain at day 7. In contrast, E149A-APC treatment worsened neurological outcome determined by foot fault tests and forelimb use asymmetry tests, and increased significantly by 44% and 60% infarct and edema volume, respectively (P<0.05). At 7days after treatment, E149A-APC compared to vehicle or 5A-APC notably increased by ~5-fold the hemoglobin level in the ischemic hemisphere suggesting it provoked significant intracerebral bleeding. Thus, the enhanced anticoagulant activity of E149A-APC increased post-ischemic accumulation of neurotoxic erythrocyte-derived hemoglobin which likely worsened the neurological and neuropathological outcomes after stroke. Our data emphasize that APC's cytoprotective activities, but not its anticoagulant activity, are key for APC neuroprotection after transient ischemic stroke.

Increased plasma soluble thrombomodulin levels in cardioembolic stroke

Soluble thrombomodulin (sTM) has been proposed as a potential marker of ischemic stroke. Results from previous studies remain controversial among different populations. We performed an analysis of plasma levels of sTM in Thai patients with acute ischemic stroke and determined whether sTM levels correlate with stroke subtypes, severity, and risk factors. Ninety-three patients and 76 controls were enrolled. Blood samples were obtained within 24 hours after stroke onset. Plasma sTM levels, measured using quantitative enzyme-linked immunosorbent assay, were significantly higher in patients than controls (P < .005), with the mean ± standard deviation (SD) levels of 3.08 ± 1.05 and 2.57 ± 1.15 ng/mL, respectively. Plasma levels of sTM in patients with cardioembolic subtype were significantly higher than in patients with other stroke subtypes, with the mean ± SD levels of 3.79 ± 1.26, 2.38 ± 0.68 (P < .009), and 2.38 ± 0.44 (P < .05) ng/mL for cardioembolism, large artery atherosclerosis, and small artery occlusion, respectively. Plasma sTM levels were not associated with stroke severity and risk factors of stroke; however, there was a slight relationship between high sTM levels and the presence of atrial fibrillation in the patient group. In conclusion, plasma sTM levels were increased in Thai patients with cardioembolic stroke and may be a potential marker during the acute phase.

The promise of protein C

Protein C, a vitamin K-dependent serine protease zymogen that circulates in plasma, is converted by limited proteolysis to activated protein C (APC) by the thrombin-thrombomodulin complex. APC exerts anticoagulant, antiinflammatory, cytoprotective, and antiapoptotic activities. Recombinant APC therapy reduces mortality in severe sepsis patients. This review summarizes data from clinical observations, from in vitro studies, and from animal models of focal ischemic injury that provide a compelling rationale for clinical trials of APC for ischemic stroke.

Antithrombotic agents in the treatment of severe sepsis

Sepsis, a systemic inflammatory syndrome, is a response to infection and when associated with multiple organ dysfunction is termed severe sepsis. It remains a leading cause of mortality in the critically ill. The response to the invading microorganisms may be considered as a balance between a pro-inflammatory and an anti-inflammatory reaction. While an inadequate pro-inflammatory reaction and a strong anti-inflammatory response could lead to overwhelming infection and the death of the patient, a strong and uncontrolled pro-inflammatory response, manifested by the release of pro-inflammatory mediators may lead to microvascular thrombosis and multiple organ failure. Endotoxin triggers sepsis via the release of various mediators such as tumour necrosis factor-alpha and interleukin-1 (IL-1). These cytokines activate the complement and coagulation systems, release adhesion molecules, prostaglandins, leukotrienes, reactive oxygen species and nitric oxide. Other mediators involved in the sepsis syndrome include IL-1, -6 and -8; arachidonic acid metabolites; platelet activating factor; histamine; bradykinin; angiotensin; complement components and vasoactive intestinal peptide. These pro-inflammatory responses are counteracted by IL-10. Most of the trials targeting the different mediators of the pro-inflammatory response have failed due to a lack of correct definition of sepsis. Understanding the exact pathophysiology of the disease will enable more advanced treatment options. Targeting the coagulation system with various anticoagulant agents including, activated protein C, and tissue factor pathway inhibitor (TFPI) is a rational approach. Many clinical trials have been conducted to evaluate these agents in severe sepsis. While trials on antithrombin and TFPI were not so successful, the double-blind, placebo-controlled, Phase III trial of recombinant human activated Protein C Worldwide Evaluation in Severe Sepsis (PROWESS) was successful, creating a significant decrease in mortality when compared to the placebo group. A better understanding of the pathophysiologic mechanism of severe sepsis will provide better treatment options, and combination antithrombotic treatment may provide a multipronged approach for the treatment of severe sepsis.

Activated protein C and ischemic stroke

OBJECTIVE

To summarize clinical observations, animal model experimentation, and in vitro studies that advance knowledge of the protein C system, including activated protein C (APC), in the setting of ischemic stroke.

DATA SOURCE

Narrative review of selected published primary basic and clinical literature from MEDLINE for 2000-2003.

DATA SYNTHESIS

Low levels of plasma APC and a poor response to APC in clotting assays may be markers or risk factors for ischemic stroke. Ischemia during routine endarterectomy causes APC generation in the affected region of the human brain. The prospective epidemiologic Atherosclerosis Risk in Communities (ARIC) study reported that plasma protein C may be protective for ischemic stroke. In murine models of focal cerebral ischemia, APC provided remarkable anti-inflammatory and neuroprotective effects in vivo and increased survival at 24 hrs. Recent in vitro and in vivo studies provide remarkable insights into mechanisms of the neuroprotective activities of APC. Independent of its well-known anticoagulant activity, APC acts directly on cells and alters gene expression profiles, inhibits apoptosis, and down-regulates inflammation. These effects require protease-activated receptor-1 and the endothelial protein C receptor. In an in vitro model involving hypoxia-induced apoptosis of human brain endothelial cells, protease-activated receptor-1 and endothelial protein C receptor were required for APC to exert its anti-apoptotic effects. In these cells, APC blunts hypoxia-induced increases in p53 messenger RNA and protein, reduces pro-apoptotic Bax, and increases anti-apoptotic Bcl-2, thereby inhibiting mitochondrial-dependent apoptosis. Murine ischemic stroke model studies have provided in vivo evidence for the physiologic roles of protease-activated receptor-1 and endothelial protein C receptor in the neuroprotective activities of APC. Because the low doses required for recombinant murine APC to provide neuroprotection do not cause observable anticoagulant effects, the in vivo neuroprotective action of APC seems, at least in part, to be independent of its anticoagulant activity and is likely to involve its anti-apoptotic activity.

CONCLUSIONS

There is compelling evidence that ischemic stroke is an attractive target for therapy with APC.

Activation of protein C during reperfusion in clinical liver transplantation

BACKGROUND

Activated protein C (APC) exhibits anticoagulant and antiinflammatory properties. We studied the kinetics and magnitude of protein C activation in clinical liver transplantation and the interaction of this activation with neutrophil and monocyte activation.

METHODS

In 10 patients undergoing liver transplantation, we measured plasma protein C and APC levels, neutrophil and monocyte CD11b and L-selectin expression, and leukocyte differential counts pre-, intra-, and postoperatively. Samples of blood entering and leaving the liver were obtained simultaneously to assess changes across the liver.

RESULTS

Protein C level was low preoperatively (65%, range 39%-141%) and remained low throughout surgery. Compared with the preoperative level (107%, range 78%-161%), APC level increased during liver reperfusion (471%, range 183%-917%, P=0.05). A transhepatic decrease in protein C level (-16%, range -45%-5%, P=0.007), but not in APC level, occurred during initial liver reperfusion. At the same time, neutrophil and monocyte activation took place in the liver.

CONCLUSIONS

Despite protein C deficiency, patients with liver insufficiency are able to maintain normal APC levels. During reperfusion, protein C consumption occurs in the liver without concomitant hepatic release of APC, indicating a shortage of APC in the reperfused liver. The process consuming protein C and APC may be related to the simultaneous ongoing neutrophil and monocyte activation within the liver graft, indicating a regulatory role for APC in inflammation.

Reduced thrombomodulin in human peripheral nerve microvasculature

Thrombomodulin (TM), a vascular endothelial receptor, terminates the actions of thrombin and accelerates activated protein C formation. TM is ubiquitous throughout the systemic microcirculation but is reduced in brain regions predisposed to lacunar infarction. We investigated whether TM is present within human nerve and differentially expressed according to vessel caliber and proximity to the blood-nerve barrier. Vascular endothelial TM was detected on sural nerve biopsies with immunohistochemistry. The proportion of TM-positive microvessels was expressed relative to total von Willebrand factor (vWF)-positive vessels. Although vWF was detectable in all microvessels, TM expression was absent from the perineurial vessels. TM was detected in 47% (15-80%, 95% confidence level) of larger epineurial arterioles, in 43% (30-61%) of smaller epineurial vessels, and in 30% (19-47%) of endoneurial vessels. These findings demonstrate that TM is present in human nerve microvasculature but is regionally deficient in proximity to the blood-nerve barrier, which may predispose nerve to microvascular ischemia in inflammatory/prothrombotic conditions.

Activated protein C: potential therapy for severe sepsis, thrombosis, and stroke

Activated protein C (APC) reduced all-cause 28-day mortality by 19% in patients with severe sepsis (sepsis associated with acute organ dysfunction) in the Protein C Evaluation in Severe Sepsis (PROWESS) trial, leading to recent approval of recombinant APC for treatment of this condition in adults. This review summarizes current knowledge derived from studies of a variety of animal models in which infused human APC demonstrated beneficial activities. Based on in vivo and also in vitro data, APC manifests antithrombotic, profibrinolytic, anti-inflammatory, and antiapoptotic activities. APC is a normal circulating component of plasma, derived from the protein C zymogen, and is thus a natural endogenous protective homeostatic factor. Because of its multiple activities, APC has a potential role in the treatment of complex and challenging medical disorders, including thrombosis and stroke.

Thrombomodulin deficiency in human diabetic nerve microvasculature

Human diabetic neuropathy is multifactorial in etiology, with ischemia as a final common pathology. Although impaired vascular endothelial cell function in diabetic microvascular injury is established, the role of thrombomodulin (TM)-dependent protein C antithrombotic mechanism in the pathogenesis of neuropathy is unclear. This neuropathologic case-control study investigated whether vascular endothelial TM expression is deficient in peripheral nerve microvessels in diabetic neuropathy. Sural nerve biopsies from 7 patients with diabetic neuropathy and 10 with axonal neuropathy without vasculopathy were immunostained with anti-TM and anti-von Willebrand factor (vWF; an endothelial cell marker) antibodies. The proportion of TM-positive microvessels was expressed relative to total vWF-staining vessels, according to vessel caliber and regional distribution within the nerve. In diabetic nerves compared with reference controls, the proportion of TM-positive endoneurial microvessels was 15-fold lower (0.02 vs. 0.30 in diabetic nerves vs. controls, P < 0.004), and the proportion of small-caliber epineurial microvessels was 10-fold lower (0.04 vs. 0.43, P < 0.001). No TM expression was detected at the perineurium in diabetic or control nerves. We demonstrate a substantial reduction of vascular endothelial TM expression throughout human diabetic neuropathy. These findings suggest that an impaired native TM-dependent protein C antithrombotic mechanism may contribute to microvascular ischemia in the pathogenesis of diabetic neuropathy.

Activated protein C versus protein C in severe sepsis

OBJECTIVE

To delineate critical differences between activated protein C (APC) and its precursor, protein C, with regard to plasma levels in health and in severe sepsis, and to discuss the implications of these differences as they relate to treatment strategies in patients with severe sepsis. DATA SOURCE/STUDY SELECTION: Published literature including abstracts, manuscripts, and review articles reporting studies in both experimental animal models and humans that provide an understanding of the relationship and the critical differences between circulating levels of APC and protein C.

DATA EXTRACTION AND SYNTHESIS

The protein C pathway represents one of the major regulatory systems of hemostasis, exhibiting antithrombotic, profibrinolytic and anti-inflammatory properties. This pathway also plays a critical role in the pathophysiology of severe sepsis. Central to this pathway is the vitamin K-dependent serine protease, APC, and its precursor, protein C. The conversion of protein C to APC is dependent on the complex of thrombin and thrombomodulin, an integral endothelial surface receptor. The conversion of protein C to APC is further augmented by another endothelial surface protein, the endothelial protein C receptor. There are limited published data on APC levels in health and disease, probably due to the complexity of the assay methodology for measuring APC and the absence of commercially available diagnostic kits. In animals and humans with normal functioning endothelium, circulating levels of APC (1-3 ng/mL) are positively correlated with protein C (4000-5000 ng/mL) concentration and the amount of thrombin generated. In patients with severe sepsis, there is a generalized endothelial dysfunction, contributing to multiple organ failure with increased morbidity and mortality. Persistently low protein C levels are related to poor prognosis. Key to understanding the treatment strategy with APC or protein C is knowledge of the functional status of the endothelium and, specifically, whether the microvasculature in patients with severe sepsis can support the conversion of protein C to APC. To date, only APC (drotrecogin alfa [activated]) has been shown to reduce mortality in severe sepsis in a large, phase 3, placebo-controlled, double-blind international trial. In contrast, no data, other than open-label case studies, are available for evaluation of the effects of protein C in the treatment of severe sepsis.

CONCLUSION

The limited data available indicate that lower levels of protein C in sepsis occur in the absence of appreciable conversion to APC. These observations indicate that treatment with APC may be more efficacious than protein C in severe sepsis, where generalized endothelial dysfunction may impair conversion of protein C to APC. Additional research is required to confirm these observations.

Brain injury and cerebrovascular fibrin deposition correlate with reduced antithrombotic brain capillary functions in a hypertensive stroke model

Hemostasis factors may influence the pathophysiology of stroke. The role of brain hemostasis in ischemic hypertensive brain injury is not known. We studied ischemic injury in spontaneously hypertensive rats in relation to cerebrovascular fibrin deposition and activity of different hemostasis factors in brain microcirculation. In spontaneously hypertensive rats subjected to transient middle cerebral artery occlusion versus normotensive Wistar-Kyoto (W-K) rats, infarct and edema volumes were increased by 6.1-fold (P < 0.001) and 5.8-fold (P < 0.001), respectively, the cerebral blood flow (CBF) reduced during middle cerebral artery occlusion (MCAO) by 55% (P < 0.01), motor neurologic score increased by 6.9-fold (P < 0.01), and cerebrovascular fibrin deposition increased by 6.8-fold (P < 0.01). Under basal conditions, brain capillary protein C activation and tissue plasminogen activator activity were reduced in spontaneously hypertensive rats compared with Wistar-Kyoto rats by 11.8-fold (P < 0.001) and 5.1-fold (P < 0.001), respectively, and the plasminogen activator inhibitor-1 antigen and tissue factor activity were increased by 154-fold (P < 0.00001) and 74% (P < 0.01), respectively. We suggest that hypertension reduces antithrombotic mechanisms in brain microcirculation, which may enhance cerebrovascular fibrin deposition and microvascular obstructions during transient focal cerebral ischemia, which results in greater neuronal injury.